View source

From Colwiki.org

for Devices and Linux File System Management
You do not have permission to do that, for the following reason:
The action you have requested is limited to users in the group user.
You can view and copy the source of this page:
{{Outcomes|In this Module you will learn how to:
*Create partitions and File Systems
*File Permissions
*Monitoring Disk Usage
*Control File System Mounting and Unmounting}}

=Devices and Linux File System Management=
[[Image:Terminology.png]]      Partitions:

A hard disk partition is a defined storage space on a hard drive. Disk partitioning is the act or practice of dividing the storage space of a hard disk drive into separate data areas known as partitions.     

=Creating Linux Partitions=

One has the choice to associate a piece of hardware (or resource) to a directory.  For example the root directory “/” which is more or less like the C:\ drive for DOS could correspond to the /dev/hda2 partition, and the subdirectory /boot could correspond to the partition /dev/hda3. “/dev/hda3 is said to be mounted on /boot”. The directory on which a block device is mounted is then 
ntuitive installation tools such as DiskDruid.Finally, for beginners and busy sysadmin’s, the latest Linux distributions will automatically assign a partition scheme. 

Once the operating system is installed you can use the '''fdisk''' utility to configure new partitions. We will next look at the basic syntax for '''fdisk'''

*Start partitioning the first hard drive:  
   '''fdisk  /dev/hda'''
*Type '''m''' for help. Then create a new partition with '''n'''.
*To write the changes to disk type '''w'''.
*'''REBOOT'''

These four points outline the steps you would follow to create new partitions. The last point is often overlooked. This forces the partition table in the master boot record MBR to be reread.

'''NOTICE'''

''You need to create a filesystem on a new partition with '''mkfs''' or '''''mke2fs''''' before using it''

This ends the survey of available partitioning tools. We next take a look at bootloaders.

=Managed Devices=
At boot time the /etc/fstab file assigns mount points for block devices.

The ''/etc/fstab'' format

  '''device    mount-point     fstype       options   dump-number  fsck-number'''

Sample '''''/etc/fstab'''''

  '''LABEL=/            /               ext2    defaults                1 1'''
  '''LABEL=/boot        /boot           ext2    defaults                1 2'''
  '''LABEL=/home        /home           ext3    defaults                1 2'''
  '''/dev/fd0           /mnt/floppy     auto    noauto,owner            0 0'''
  '''LABEL=/usr         /usr            ext2    defaults                1 2'''
  '''LABEL=/var         /var            ext3    defaults                1 2'''
  '''none               /proc           proc    defaults                0 0'''
  '''none               /dev/shm        tmpfs   defaults                0 0'''
  '''none               /dev/pts        devpts  gid=5,mode=620          0 0'''
  '''/dev/hdc9          swap,pri=-1     swap    defaults                0 0'''
  '''/dev/cdrom         /mnt/cdrom      iso9660 noauto,owner,kudzu,ro   0 0'''

On a running system the '''/etc/fstab''' file also acts as a shortcut for assigning a resource to a specific directory. For example:

  ''mount /dev/cdrom''

The '''mount''' utility reads '''fstab''' and deduces where to mount the resource. Notice that some of the devices are accessed using a label. Labels are assigned to devices with the '''tune2fs''' tool:

  ''tune2fs -L /usr/local /dev/hdb12''

'''Option summary for mount:'''

  ''rw,ro -  read-write and read-only''
  ''users -  the device can be read and unmounted by all users'' 
  ''user  -  the device can unmounted only by the user''
  ''owner -  the device will change it's permission and belong to the user that mounted it''
  ''usrquota - start user quotas on the device''
  ''grpquota - start group quotas on the device''
'''NOTICE'''

''Remember that '''mount -a''' will mount all filesytems in '''/etc/fstab''' that have not been mounted and do not have the option '''noauto'''''

=File Permissions=
[[Image:Permission.png]]

Permissions can be acted upon with '''chmod'''. There are 3 categories of ownership for each file and directory:

The symbolic values for the owner fields:

'''u:''' a valid user with an entry in /etc/passwd
'''g:''' a valid group with an entry in /etc/group
'''o:''' other

Example:
  ''-rw-rw-r--    1 jade   sales         24880 Oct 25 17:28 libcgic.a''

Changing Permission:
  ''chmod g=r,o-r libcgic.a'' 
  ''chmod g+w  libcgic.a''

Changing user and group:
  ''chown  root  libcgic.a'' 
  ''chgrp  apache libcgic.a''

'''NOTICE'''

''A useful option for chmod, chown and chgrp is –R which recursively changes ownership and permissions through all files and directories indicated.''


'''Symbolic and octal notation'''

Permissions can be read=r, write=w and execute=x. The octal values of these permissions are listed in the next table.

''Octal and symbolic permissions.''

  '''Symolic           octal'''
  ''read               4''
  ''write              2''
  ''execute            1''
Permissions apply to the user, the group and to others. An item has a set of 3 grouped permissions for each of these categories.

''How to read a 755 or -rwxr-xr-x permission ''
  '''user        group       other'''
  ''rwx         r_x         r_x''
  ''4+2+1=7     4+1=5       4+1=5''

'''The standard permission'''
 
UNIX systems create files and directories with standard permissions as follows:

'''Standard permission for:'''
  ''Files         666         -rw-rw-rw-''
  ''Directories   777         -rwxrwxrwx''

'''Umask'''

Every user has a defined '''umask''' that alters the standard permissions. The '''umask''' has an octal value and is subtracted(*) from the octal standard permissions to give the files permission (this permission doesn't have a name and could be called the file's effective permission).(*) While subtraction works in most cases, it should be noted that technically the standard permissions and the umask are combined as follows:

Final Permissions = Standard Permissions (logical AND)  (NOT)Umask

On systems where users belong to separate groups, the umask can have a value of 002. For systems which place all users in the users group, the umask is likely to be 022.

'''SUID permissions'''

It is possible for root to give users permission to execute programs they would usually be unable to. This permission is the SUID permission with a symbolic value s or a numerical value 4000. 
For example root can write a C shell script that executes a program and set the SUID of the script with chmod 4777 script or chmod u+s script. (NB Bourne and Bash scripts do not honour SUID bits set on the script files.)

'''Examples:'''

  ''chmod 4755 /bin/cat''
  ''chmod u+s /bin/grep''

'''SGID permissions'''

The SGID is a similar permission set for group members. The symbolic value is '''s''' and the '''octal value of 2000'''.

Setting SGID on a directory changes the group ownership used for files subsequently created in that directory to the directories group ownership. No need to use newgrp to change the effective group of the process prior to file creation.

'''Examples:'''

  ''chmod 2755 /home/data''
  ''chmod g+s /bin/wc''

'''The sticky bit'''

The sticky bit permission with value 1000 has the following effect:

#Applied to a directory it prevents users from deleting files unless they are the owner (ideal for directories shared by a group)
#Applied to a file this used to cause the file or executable to be loaded into memory and caused later access or execution to be faster. The symbolic value for an executable file is t while for a non executable file this is T. As file system caching is more generic and faster, file sticky bits tend not be supported any more.

  ''chmod 1666 /data/store.txt''
  ''chmod o+t /bin/bash''

{{Summary|In this module you learned how to work with partitions and allocate the appropriate rights to files that you will work with. Linux is very particular on the rights issues. The appropriate ownership is required to be able to execute and run applications within Linux.

It is important that you understand how Linux maintains and manages its access rights.}}



{{Assignment|Create 2 new partitions (larger than 50M) on the /dev/hda device using fdisk. HINT: To create a new partition type n. The partition type defaults to 83 (Linux)

*To write the new partition table type w. 
*The partition table needs to be read: REBOOT the computer!

Format the first partition using the ext2 filesystem type and the second with ''reiserfs''. 
HINT: The ''mkfs'' tool is a front for mkfs.ext2 or mkfs.reiserfs, etc. The syntax is

  ''mkfs –t <fstype> <device>''

Make directories in /mnt and mount the new partitions
	
  ''mkdir /mnt/ext2''
  ''mkdir /mnt/reiserfs''

Use ''mount'' to verify which devices are mounted. The permissions set in fstab are visible too.

Use ''df'' to check the total number of blocks used. The –k option will convert the number of blocks in kilobytes (the default block size for ext2)

Run ''fsck'' on one of the newly created filesystems. The fsck utility is a front for fsck.ext2, fsck.ext3, fsck.reiserfs, etc. The syntax is:

  ''fsck     <device>''
 
Notice that there are no tools to create ext3 formated partitions. In fact the ext3 format is the same as the ext2 format with a journal added. These are the steps:

  ''mke2fs /dev/hda10''
  ''tune2fs –j /dev/hda10''

At this stage the system has added a journal to the '''''/dev/hda10''''' partition, making it an ext3 formated partition. This process is non-destructive and reversible. If you mount an ext3 as an ext2 filesystem, the .journal file will be erased. You can add it again with '''''tune2fs'''''.
.}}

[[Image:somerights20.png]] This work is licenced under a [http://creativecommons.org/licenses/by-sa/3.0/ Creative Commons - By Attribution  Licence - Share Alike] License.
Templates used on this page:
Return to Devices and Linux File System Management.
Retrieved from "http://colwiki.org/Devices_and_Linux_File_System_Management"
View source

From Colwiki.org

Views

Personal tools

Navigation

News & Events

Community

download

Search

Toolbox
